An engine for use in vehicles such as an automotive vehicle includes a cam shaft for operatively opening/closing intake/exhaust valves. The cam shaft is a shaft body having a plurality of cams arranged in an axial direction thereof and is disposed in a valve operating chamber enclosed by a cylinder head and a head cover of an engine. The cam shaft is supported rotatably relative to the cylinder head by means of a cam shaft supporting apparatus including a plurality of sliding bearings axially arranged with regular spacing.
However, the sliding bearing of the cam shaft supporting apparatus may sometimes have a relatively great frictional resistance in a state where an engine speed is low, so as to be increased in rotary torque. The sliding bearing constitutes one causative factor to decrease the mileage performance of the engine. In this connection, Japanese Unexamined Utility Model Publication No. H6(1994)-8704 and Japanese Unexamined Patent Publication No. H4(1992)-12102 have disclosed structures wherein the cam shaft is supported by rolling bearings in order to improve the mileage performance of the engine. In this manner, the approaches have been made to decrease the frictional resistance for decreasing the rotary torque.
The conventional structures for rotatably supporting the cam shaft in the cylinder head include one wherein the sliding bearing is used as set forth in Japanese Unexamined Patent Publication No. H8(1996)-218817 and one wherein the rolling bearing is used as set forth in Japanese Unexamined Utility Model Publication No. H5(1993)-6104. In the case where the sliding bearing is used, a lubricant is fed to an outer periphery of the sliding bearing via an oil feeding hole formed at the cylinder head and then is delivered to space between an outer periphery of the shaft and an inner periphery (journal surface) of the sliding bearing via a radial through-hole formed in the sliding bearing. However, the sliding bearing so lubricated has a greater frictional resistance with respect to the shaft as compared with the case where the rolling bearing is used. It is therefore more favorable to use the rolling bearing in the light of achieving a greater reduction of the frictional resistance.